Many communication systems employ timing structures for managing communications between devices. Time can be defined in any number of units and subunits where the communication equipment applies the timing to transmission and reception. For example, some systems divide time into at least frames, subframes, timeslots, and symbol times to form the physical channel timing structure. A physical channel structure may also involve dividing and organizing the available frequency bandwidth. As a result, a defined physical channel structure typically includes dividing the frequency and time resources into units and subunits to allow efficient management and use of time-frequency communication resource.
The frequency spectrum used by a particular communication system may be licensed or unlicensed. Licensed frequency spectrum (frequency band) is licensed to a system operator by a government agency such as the Federal Communications Commission (FCC). An example of such licensed spectrum includes frequency bands used for cellular communication. An unlicensed frequency band is any portion of frequency spectrum that does not require a license from the government agency to communicate with the unlicensed frequency band. Equipment operating within the unlicensed band, however, typically must adhere to regulations and/or communication standards. An example of unlicensed frequency spectrum includes frequency bands used for Institute of Electrical and Electronics Engineers (IEEE) 802.11 communication.